• by Mark Walsh  @markfwal, February 1, 2010

Facebook maintained steady growth into the new year, adding 5 million U.S. users in January for a total of 108 million.

Young men and women led the way among demographic segments. The single biggest gain was among men ages 26 to 34, with more than 600,000 joining in January, according to new data from Inside Facebook. Right behind that group were women 18 to 25, at 581,000.

But older users remain among the fastest-growing populations on Facebook. Men 45 to 54 showed the highest growth rate last month -- with 472,000 joining, or 10.5%, followed closely by women 55 to 65, at 9.4% (391,000). That compares to growth rates, respectively, for the two groups of 0.8% and 1.6%. Why the senior surge?

Inside Facebook surmises that young people went home at Christmas and made their parents sign up. "Facebook saw massive traffic spikes around Christmas and New Year's, according to Hitwise, and we wonder if this was when thousands of Facebook evangelists fired up the computer -- or laptop, or netbook, or iPhone -- and taught their relatives how Facebook works," noted Eric Eldon.

While it may be hard to imagine college-age kids trying to get their parents onto Facebook, one commenter suggested that empty nesters are increasingly turning to the site to maintain bonds with their children. Inside Facebook identified women 55 and over as the site's fastest-growing group a year ago, increasing 175% during the prior 120 days. As that demographic has grown to 4.5 million, that rate has slowed considerably but is still respectable.

Overall, users 18 to 25 represent the largest share of Facebook members -- at 29%, followed by 26-34 (23%); 35-44 (18%); 45-54 (12%); 13-17 (11%); and 55-65 (7%).

According to comScore, Facebook's monthly audience more than doubled from 54.5 million U.S. unique visitors to 112 million unique visitors during 2009.